Linear low density polyethylene (LLDPE), which has a density from 0.916 to 0.940 g/mL, has penetrated all traditional markets for polyethylene, including film, molding, pipe, and wire and cable. Due to its strength and toughness, LLDPE has been largely used in the film market, such as produce bags, shopping bags, garbage bags, diaper liners, and stretch wrap. LLDPE has been primarily made with conventional Ziegler catalysts. It is typically produced by copolymerization of ethylene with a long chain .alpha.-olefin such as 1-butene, 1-hexene, or 1-octene.
In the early 1980's, Kaminsky discovered a new class of olefin polymerization catalysts known as metallocenes (see U.S. Pat. Nos. 4,404,344 and 4,431,788). A metallocene catalyst consists of a transition metal compound that has one or more cyclopentadienyl (Cp) ligands. Unlike Ziegler catalysts, metallocene catalysts are usually soluble in olefins or polymerization solvents and give homogeneous polymerization systems. Since these catalysts have a single reactive site (compared with multiple reactive sites of Ziegler catalysts), they are also called "single-site" catalysts. Metallocene catalysts are more reactive than conventional Ziegler catalysts, and they produce polymers with narrower molecular weight distributions. Because single-site catalysts enhance incorporation of long chain .alpha.-olefin comonomers into polyethylene, they are of particular interest in the production of LLDPE.
Over the last decade, non-metallocene single-site catalysts have also been developed rapidly. Non-metallocene single-site catalysts contain non-Cp ligands, which are usually heteroatomic ligands, e.g., boraaryl, azaborolinyl, pyridinyl, pyrrolyl, indolyl, carbazolyl, or quinolinyl groups. The development of non-metallocene single-site catalysts has provided the polyolefin industry with more choices of catalysts and opportunities for optimizing the products or production processes.
Non-metallocene single-site catalysts have most of the characteristics of metallocene catalysts, including high activity. However, they produce polyethylenes that have relatively high density. For example, boraaryl-based single-site catalysts produce polyethylenes that have densities from about 0.93 to about 0.97 g/mL (see U.S. Pat. No. 5,554,775). It is of significant interest to further lower the density of the polyethylenes produced with non-metallocene single-site catalysts.